1. Field of the Invention
The present invention relates to the transfer of call legs within a telecommunication network. While the invention is applicable to many different types of networks, it particularly lends itself to use in the transfer of B channels within an integrated services digital network (ISDN) telecommunication environment. The invention is applicable to B channel transfers where the B channels are part of a primary rate interface (PRI) or ISDN user part (ISUP) based trunk network.
2. Description of the Problem Solved
Businesses and enterprises that use the public telecommunication network typically require services such as call transfer, conference calling, and call forwarding. In many cases the services are supplied by service nodes which are connected to the public network switch which serves the particular enterprise. Examples of service nodes include voicemail systems and international call-back servers. For purposes of this disclosure, we also consider a PBX or any other type of privately owned telecommunication device to be a service node. In order to supply the services required by the enterprise, the switches and service nodes often have to maintain multiple call legs for a given call. Each call leg between a given switch and a given service node is connected via a channel which is part of an interface. In an ISDN environment each channel is a B channel, and the interface is either a primary rate interface or an ISUP controlled trunk.
There are numerous situations where multiple call legs might be used. As an example, consider a user who dials into a voicemail system. Typically, a voicemail system provides an option to press a key and immediately and automatically call a party back who has left a message. When the user executes this function, the voicemail system dials the party over a separate connection and internally connects the user and the party within the voicemail system. The voicemail system stays in the call path so that the user can resume listening to messages when the call between the user and the called party is complete. This call path results in two B channels between the switch and the voicemail system being used to connect the user and the called party. Alternatively, the voicemail system could dial the party and connect the user and the party by making a transfer such as the B channel transfer described in published Bellcore Standard GR-2865-Core, Issue 2, May 1997, which is incorporated herein by reference. This transfer frees up the two B channels between the switch and the voicemail system, but at a cost to the user. When the call between the user and the called party is complete, the user must re-dial the voicemail system to resume listening to voicemail messages.
The problem created by the situation is that service nodes and their network connected switches are required to have extra active ports for the sole purpose of holding access to calls which the service node may need to recover in the future under certain conditions. The extra ports cost money because additional equipment must be purchased and maintained to supply the required capacity. If a personal computer is being used as a service node, extra adapter cards must be purchased. If long distance charges apply, the extra long distance costs can be considerable. What is needed is a way to release and conditionally retrieve a call leg which goes unused for a period of time due to a call transfer or similar event. Ideally, call retrieval should be able to be based on the occurrence of one or more of a variety of events.